This invention relates to data communication systems, and more particularly to a clock-recovery technique for a serial data transmission system.
In U.S. Pat. Nos. 4,897,833, 4,887,076, 4,845,722, 4,777,595, 4,560,985, and 4,490,785, all assigned to Digital Equipment Corporation, assignee of this invention, computer interconnect systems are shown of the type employing packet data transfer using serial paths. These types of computer interconnect systems have been commercially used for processors and bulk memory facilities of the VAX architecture, and provide versatile systems of high performance and reliability. Other commercially-available communications networks include the so-called Ethernet system as disclosed in U.S. Pat. No. 4,063,220, issued to Metcalfe et al, and the token ring system such as the IEEE 802.5 standard and the FDDI (fibre distributed data interface) standard. Each of these communications systems, or local area network systems, employs serial data transmission.
In most local area network systems, the serial data is transmitted using a not-return-to-zero (NRZ) convention, such as Manchester encoding. The data and clock are included in the same signal, so at the receiver the clock must be recovered before the incoming data can be interpreted. A local clock is generated in frequency and phase synchronization with the clock imbedded in the datastream, and this local clock is then employed to extract the data from the incoming signals. One example of a decoder for recovering data and clock from a self-clocked encoded signal such as a Manchester-encoded signal is disclosed in U.S. Pat. No. 4,450,572, issued to Stewart et. al. and assigned to Digital Equipment Corporation.
A coding technique used in some networks employs a NRZI convention, in which case a transition (zero-to-one or one-to-zero) indicates a "one" and the absence of a transition indicates a zero. Fibre optics systems using the FDDI standard typically use this convention. Because a string of zeros produces a coded signal having no transitions, the task of recovering the clock from this signal is much more difficult, compared to a system using Manchester coding where at least one transition occurs for every bit, whether it is a one or a zero. Usually steps are taken to assure that a "one" is transmitted every set number of bits so that a clock can be recovered, but the difficulty in generating a clock when the occurrence of a transition to sync the local clock is sporadic, is apparent.